May I know the value of the burnt resistor?Thanks!!

[wkyong]

May I know the value of the burnt resistor?Thanks!!
https://img524.imageshack.us/my.php?image=burntresistortt9.png

 

[mneary]

It could be anywhere from 82R to 680R, with no other data it's hard to tell. If the resistor is burned, the transistor base is likely open. Check the 240R resistor, it dissipates more than a watt and may be damaged as well.

 

[Nigel Goodwin]

It could be much higher than that, it's really EXTREMELY uncritical, and in many cases could be left out entirely.

 

[Boncuk]

It could be much higher than that, it's really EXTREMELY uncritical, and in many cases could be left out entirely.

... because it's absolutely senseless and worthless in this position.

 

[Nigel Goodwin]

... because it's absolutely senseless and worthless in this position.

Not really, it ensures the output transistor stays turned OFF if there is slight leakage in the driver transistor - it's good practice to include it, but it's value can be almost anything higher than the lower resistor - preferably a LOT higher though.

Quick thought - if that resistor has burnt up, it can only happen if the output transistor has failed (O/C BE), otherwise there's only 0.7V across it.

 

[ericgibbs]

Not really, it ensures the output transistor stays turned OFF if there is slight leakage in the driver transistor - it's good practice to include it, but it's value can be almost anything higher than the lower resistor - preferably a LOT higher though.

hi,
I agree, a 'high' base/emitter resistor is recommended.

The fact that resistor has burnt out, suggests the pnp is a short circuit [base/collector]

 

[Nigel Goodwin]

hi,
I agree, a 'high' base/emitter resistor is recommended.

The fact that resistor has burnt out, suggests the pnp is a short circuit [base/collector]

And has to be O/C base/emitter as well, or just O/C base/emitter could make it burn up as it would have far too much voltage across it.

 

[ericgibbs]

And has to be O/C base/emitter as well, or just O/C base/emitter could make it burn up as it would have far too much voltage across it.

The BD140 is an ex- BD140.!

 

[Nigel Goodwin]

The BD140 is an ex- BD140.!

It's shuffled off this mortal coil - the only reason it worked was that it was nailed to its relay!

 

[ericgibbs]

It's shuffled off this mortal coil - the only reason it worked was that it was nailed to its relay!

Its probably the back emf from the 'mortal coil' thats caused the BD140's mortality.!!!

 

[Roff]

The value of the base-emitter resistance affects the turn-off time of the transistor. See below.

 

[Nigel Goodwin]

The value of the base-emitter resistance affects the turn-off time of the transistor. See below.

It's of VERY little concern though, if it was of concern you would make better arrangements than that so it turned off much faster still.

 

[Roff]

It's of VERY little concern though, if it was of concern you would make better arrangements than that so it turned off much faster still.

I was taking issue with those who said that Rbe is unnecessary, or at least irrelevant. It may or may not be important in this particular circuit.